Method, apparatus and system for improving cell search and synchronization in a cellular communication system by using non-circularity of signal statistics

ABSTRACT

There are provided measures for cell search and synchronization. Such measures may exemplarily comprise acquiring an observation signal for a carrier signal on a carrier which is under consideration for synchronization with a desired cellular system, calculating a power measure of the observation signal, which indicates a received power of said carrier signal, calculating a non-circularity measure of the observation signal, which indicates a non-circularity of said carrier signal, and calculating a ranking measure, which indicates an applicability of said carrier for synchronization with the desired cellular system, based on the calculated power measure and the calculated non-circularity measure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/301,924, filed on Nov. 22, 2011, which claims the benefit of andpriority to United Kingdom patent application number 1119437.0, filed onNov. 10, 2011.

FIELD OF THE INVENTION

The present invention relates to cell search and synchronization. Morespecifically, the present invention relates to measures (includingmethods, apparatuses and computer program products) for cell search andsynchronization in a cellular system using non-circularity of signalstatistics.

BACKGROUND

In the field of mobile communication systems, particularly cellularcommunication systems, one issue relates to cell search andsynchronization. Such cell search and synchronization is to be performedby any terminal for connecting to a cellular system and for ensuringterminal mobility within a cellular system, i.e. a cell thereof, eitherwhen powering up the terminal or when transiting from an idle mode to aconnected mode or when transiting from one cell to another cell.

The speed with which cell search and synchronization can be performed bya terminal is important for both end-user experience and the resultingpower consumption of the terminal. For the end user, increasedsynchronization time and power consumption means reduced standby timeand significantly prolonged establishment of the cellular connectionwith the cellular system. Therefore, the (initial) cell search andsynchronization is an important aspect of the network performance andend-user experience. This is why it is typically considered to becritical to ensure that the terminal's (initial) synchronization time isa low as possible as well as the idle mode power consumption.

Conventionally, cell search and synchronization procedures typicallyrely on a power-based ranking of carrier signals being broadcasted onspecific carriers (such as synchronization signals being broadcasted onspecific synchronization channels), which are received at a terminaltrying to re-/connect to a cellular system. Specifically, it is knownthat the terminal calculates RSSI measures for available carrier signalsof specific carriers, ranks the carriers based on the RSSI measuresthereof, and executes a cell search and synchronization procedure basedon the carrier ranking starting with the carrier having the highestranking.

Such conventional cell search and synchronization procedures areefficient in terms of synchronization time and power consumption as longas it is ensured that all available (i.e. received) carriers or carriersignals are relevant for the cell search and synchronization purpose.Hence, such conventional cell search and synchronization procedures aresufficient in deployment scenarios in which only a single cellularsystem operates on a specific frequency range/band which is monitored byterminals for connecting to this cellular system.

However, in recent and future mobile communication systems, particularlycellular communication systems, there will increasingly be in practicedeployment scenarios in which multiple cellular systems operate on aspecific frequency range/band. For example, such deployment scenariosare resulting from 3G frequency re-farming activities, e.g. theintroduction of WCDMA services on the GSM band, LTE frequency re-farmingactivities, and the like. Hence, a terminal in such deployment scenariois likely to experience networks with one or more of GSM, 3G, 3.5G,CDMA, WCDMA, TD-SCDMA, and LTE/LTE-A carriers co-existing in a specificfrequency range/band.

Such deployment scenarios are problematic in terms of efficiency ofconventional cell search and synchronization procedures. This isessentially because the carriers and carrier signals may not bedistinguished regarding their origin from or belonging to a specificcellular system. Hence, the RSSI-ranked carrier list will be disruptedby carriers of non-desired cellular systems, i.e. those cellular systemswhich the terminal does actually not desire to connect to. As a resultof such disrupted conventional RSSI ranking, the terminal may firstlyattempt to execute a cell search and synchronization procedure based ona carrier which originates from or belongs to a cellular system otherthan that desired to be connected to. Such attempt will naturally fail,and a further attempt of cell search and synchronization will have to beexecuted based on the next carrier in the ranking, until a relevantcarrier of the desired cellular system is reached.

When assuming that the available carriers distinctively belong tocoexisting 2G and 3G network, such disruption goes in both directions,i.e. a terminal synchronizing to a BS/BTS in a network containing both2G and 3G carriers on the same frequency band will be impacted both whenattempting synchronization to the 2G network and to the 3G network.Having additional carriers for other cellular systems will furtherincrease such disruptions.

Accordingly, every time the terminal attempts to synchronize to anon-desired cellular carrier, it wastes synchronization time and idlemode power.

Therefore, the application of conventional cell search andsynchronization procedures in deployment scenarios with multiplecarriers from different cellular systems coexisting in a given frequencyrange/band will adversely result in increased synchronization time andpower consumption.

In view thereof, there exist problems in terms of efficiency (e.g.regarding synchronization time and power consumption) in the context ofcell search and synchronization in a cellular system, in particular in adeployment scenario in which multiple carriers of different cellularsystem coexist within the same frequency range/band.

Thus, there is a need to further improve cell search and synchronizationin a cellular system.

SUMMARY

Various exemplary embodiments of the present invention aim at addressingat least part of the above issues and/or problems and drawbacks.

Various aspects of exemplary embodiments of the present invention areset out in the appended claims.

According to an exemplary aspect of the present invention, there isprovided a method comprising acquiring an observation signal for acarrier signal on a carrier which is under consideration forsynchronization with a desired cellular system, calculating a powermeasure of the observation signal, which indicates a received power ofsaid carrier signal, calculating a non-circularity measure of theobservation signal, which indicates a non-circularity of said carriersignal, and calculating a ranking measure, which indicates anapplicability of said carrier for synchronization with the desiredcellular system, based on the calculated power measure and thecalculated non-circularity measure.

According to an exemplary aspect of the present invention, there isprovided an apparatus comprising at least one processor, at least onememory including computer program code, and at least one interfaceconfigured for communication with at least another apparatus, the atleast one processor, with the at least one memory and the computerprogram code, being configured to cause the apparatus to perform:acquiring an observation signal for a carrier signal on a carrier whichis under consideration for synchronization with a desired cellularsystem, calculating a power measure of the observation signal, whichindicates a received power of said carrier signal, calculating anon-circularity measure of the observation signal, which indicates anon-circularity of said carrier signal, and calculating a rankingmeasure, which indicates an applicability of said carrier forsynchronization with the desired cellular system, based on thecalculated power measure and the calculated non-circularity measure.

According to an exemplary aspect of the present invention, there isprovided a computer program product including comprisingcomputer-executable computer program code which, when the program is runon a computer (e.g. a computer of an apparatus according to theaforementioned apparatus-related exemplary aspect of the presentinvention), is configured to cause the computer to carry out the methodaccording to the aforementioned method-related exemplary aspect of thepresent invention.

Such computer program product may comprise or be embodied as a(tangible) computer-readable (storage) medium or the like on which thecomputer-executable computer program code is stored, and/or the programmay be directly loadable into an internal memory of the computer or aprocessor thereof.

Advantageous further developments or modifications of the aforementionedexemplary aspects of the present invention are set out in the followingdescription of drawings.

By way of exemplary embodiments of the present invention, there isprovided an improvement in cell search and synchronization. Morespecifically, by way of exemplary embodiments of the present invention,there are provided measures and mechanisms for improved cell search andsynchronization in a cellular system, in particular in a deploymentscenario in which multiple carriers of different cellular system coexistwithin the same frequency range/band, using non-circularity of signalstatistics.

Thus, improvement is achieved by methods, apparatuses and computerprogram products enabling an improved cell search and synchronizationusing non-circularity of signal statistics.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete understanding of exemplary embodiments of thepresent invention, reference is now made to the following descriptiontaken in connection with the accompanying drawings in which:

FIG. 1 shows a flowchart illustrating a procedure for evaluating anapplicability of a carrier for synchronization according to exemplaryembodiments of the present invention,

FIG. 2 shows a flowchart illustrating a procedure for calculating anon-circularity measure of an observation signal according to exemplaryembodiments of the present invention,

FIG. 3 shows a flowchart illustrating a procedure for calculating aranking measure of a carrier according to exemplary embodiments of thepresent invention,

FIG. 4 shows a flowchart illustrating a cell search/synchronizationprocedure according to exemplary embodiments of the present invention,and

FIG. 5 shows a block diagram illustrating an apparatus and systemaccording to exemplary embodiments of the present invention.

DETAILED DESCRIPTION

Exemplary aspects of the present invention will be described hereinbelow. More specifically, exemplary aspects of the present are describedhereinafter with reference to particular non-limiting examples and towhat are presently considered to be conceivable embodiments of thepresent invention. A person skilled in the art will appreciate that theinvention is by no means limited to these examples, and may be morebroadly applied.

It is to be noted that the following exemplary description mainly refersto specifications being used as non-limiting examples for certainexemplary network configurations and deployments. In particular, for theapplicability of thus described exemplary aspects and embodiments, GSM-and 3G/3.5G/LTE-related cellular communication networks are used asnon-limiting examples. As such, the description of exemplary aspects andembodiments given herein specifically refers to terminology which isdirectly related thereto. Such terminology is only used in the contextof the presented non-limiting examples, and does naturally not limit theinvention in any way. Rather, any other communication systems, networkconfigurations or system deployments, etc. may also be utilized as longas compliant with the features described herein.

Hereinafter, various embodiments and implementations of the presentinvention and its aspects or embodiments are described using severalalternatives. It is generally noted that, according to certain needs andconstraints, all of the described alternatives may be provided alone orin any conceivable combination (also including combinations ofindividual features of the various alternatives).

According to exemplary embodiments of the present invention, in generalterms, there are provided mechanisms, measures and means for cell searchand synchronization in a cellular system, in particular in a deploymentscenario in which multiple carriers of different cellular systemscoexist within the same frequency range/band, using non-circularity ofsignal statistics.

In general terms, exemplary embodiments of the present inventionbasically provide for an improved evaluation of an applicability of acarrier for synchronization with a desired cellular system and, thus, animproved ranking of carriers in terms of their applicability forsynchronization with the desired cellular system.

According to exemplary embodiments of the present invention, an improvedranking measure is proposed for the context of cell search andsynchronization, which modifies a power measure relating to a carriersignal by means of a non-circularity measure relating to the carriersignal. Stated in other words, the ranking measure is not a conventionalpower measure but a conventional power measure being modified byincorporating knowledge of non-circular statistics relating to thecarrier signal. Such non-circular statistics may for example show up orexist in the complex plane or any other space over which signalprecoding is done. Non-limiting examples of signals exhibiting suchnon-circular statistics include signals based on the use of real-valuedmodulations (e.g. BPSK, GMSK) or signal precoding like transmitdiversity schemes.

Accordingly, any non-circular statistics may be exploited, which mayexist in synchronization signals of a given cellular system, to eitherup- or down-prioritize specific carriers in the cell search andsynchronization procedure based on a measure of their non-circularity.As a result, the proposed ranking measure is effective for speeding upthe cell search and synchronization (and thus saving power consumption),since it enables to distinguish between carriers regarding their originfrom or belonging to a specific cellular system, and it thus enables togive priority to those carriers that are more likely to hold the signalsof the desired cellular system.

In this regard, it is utilized that carrier and/or synchronizationsignals of different cellular systems (i.e. different types, standards,releases, specifications, etc. of cellular systems) exhibit differentproperties in terms of non-/circularity, i.e. non-circular statistics inthe complex plane or any other space over which signal precoding isdone, e.g. due to the use of different modulation schemes and/or signalprecoding like transmit diversity using e.g. Alamouti precoding.

An example of a system that possesses non-circular statistics in thecomplex plane is a GSM system, which is due to the use of GMSKmodulation. Examples of systems that may possess circular statistics inthe complex plane are 3G, 3.5G, CDMA, WCDMA, TD-SCDMA, HSPA, LTE, andLTE-A systems. However, LTE and LTE-Advanced also represent examples ofsystems that possess non-circularity due to signal precoding when usingtransmit diversity schemes. Such systems as mentioned above are used asnon-limiting examples for the subsequent description.

As a result of exemplary embodiments of the present invention, a bettermeasure of the likelihood that a given carrier is e.g. a GSM signal ispossible, and this can be used to up-prioritize (i.e. privilege orfavor) the carrier in question in the GSM synchronization procedure,whereas it can be down-prioritized (i.e. penalized) in another system'ssynchronization procedure, e.g. a synchronization procedure of a 3Gsystem or the like.

Accordingly, exemplary embodiments of the present invention specificallybut not exclusively relate to 3GPP cellular systems such as GSM, 3G,3.5G, WCDMA, HSPA, and LTE, as well as non-3GPP cellular systems such asCDMA and TD-SCDMA.

Generally, exemplary embodiments of the present invention are applicableto (initial) cell search and synchronization of all cellular systemswhich are either known or expected to coexist (with e.g. GSM or LTE) onthe same frequency range/band, either now or in the future.

In the following, exemplary embodiments of the present invention aredescribed with reference to methods, procedures and functions, as wellas with reference to structural arrangements and configurations.

It is to be noted that, although processing based on linear andconjugate-linear operations is exemplarily described in the following,equivalent functionality may be accomplished based on operations workingdirectly on the real and imaginary parts of the signal.

The methods, procedures and functions described hereinafter mainlyrelate to a terminal side, which may include a mobile station (MS) or auser equipment (UE) or a modem (which may be installed as part of a MSor UE, but may be also a separate module or a chip or a chipset, whichcan be attached to various devices). Such terminal or modem isconfigured to be operable in a given frequency range/band in whichmultiple cellular systems may coexist, i.e. in which carriers ofdifferent cellular system may occur.

Generally, it is to be noted that, when reference is made herein to aterminal, MS or UE, such reference is equally applicable to a modem(which may be installed as part of a MS or UE, but may be also aseparate module or a chip or a chipset, which can be attached to variousdevices).

FIG. 1 shows a flowchart illustrating a procedure for evaluating anapplicability of a carrier for synchronization according to exemplaryembodiments of the present invention.

As shown in FIG. 1, a corresponding procedure according to exemplaryembodiments of the present invention comprises an operation of acquiring(110) an observation signal for a carrier signal on a carrier which isunder consideration for synchronization with a desired cellular system,an operation of calculating (120) a power measure of the observationsignal, which indicates a received power of the carrier signal, anoperation of calculating (130) a non-circularity measure of theobservation signal, which indicates a non-circularity of the carriersignal, and an operation of calculating (140) a ranking measure, whichindicates an applicability of the carrier for synchronization with thedesired cellular system, based on the calculated power measure and thecalculated non-circularity measure.

In the following, scalars, vectors and matrices are used, which aredenoted in a common notation. That is, non-bold non-capital lettersindicate scalars, bold non-capital letters indicate column vectors, andbold capital letters indicate matrices, and (•)*, (•)^(T) and (•)^(H)indicate the conjugate, transpose and conjugate-transpose of a vector ormatrix, respectively.

An acquisition of the observation signal for such carrier signalconsidered for an (initial) synchronization procedure may result in acomplex-valued observation signal which can be expressed byy=[y ₀ ^(T) . . . y _(M-1) ^(T)]^(T)

wherein y₀ ^(T) . . . y_(M-1) ^(T) represent the M vector-sample valuestaken over the considered observation signal. The dimensionality of thevector-samples is N, i.e. the length of y_(m) is given by N with m=[0 .. . M−1]. If necessary, the observation signal may be preconditioned ina manner that suits the target of extracting/estimating non-circularstatistics, e.g. by a suitable derotation for GSM signals.

For determining a non-circularity measure of the observation signal(i.e. the observation samples), one conceivable way assumed herein isbased on a likelihood ratio.

For example, such determination based on a likelihood ratio may berealized on a Generalized Likelihood Ratio Test (GLRT), such as thatdescribed in P. J. Schreier, L. L. Scharf, A. Hanssen, “A GeneralizedLikelihood Ratio Test for Impropriety of Complex Signals”, IEEE SignalProcessing Letters, Vol. 13, No. 7, July 2006. To compute such alikelihood ratio, it is convenient to define an augmented observationvector z containing also the conjugate observations as given byz

[y ^(T) ,y ^(H)]^(T)

Based on the observation samples, a sample covariance matrix {circumflexover (Γ)} and a complementary sample covariance matrix ĉ are given as

$\hat{\Gamma} = {\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{y_{m}y_{m}^{H}}}}$$\hat{C} = {\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{y_{m}y_{m}^{T}}}}$

Using these definitions, an augmented sample covariance matrix{circumflex over (R)} of the augmented observation vector z is given as

$\hat{R} = {{\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{z_{m}z_{m}^{H}}}} = \begin{bmatrix}\hat{\Gamma} & \hat{C} \\{\hat{C}}^{*} & {\hat{\Gamma}}^{*}\end{bmatrix}}$

Based on the augmented sample covariance matrix it, the negativelog-likelihood ratio for the hypothesis that the observations arecircularly distributed, λ

−ln(L({circumflex over (R)})), may serve as one example of anon-circularity measure. According to the aforementioned article, thisnegative log-likelihood ratio may be computed as

$\lambda\overset{\Delta}{=}{{- {\ln\left( {L\left( \hat{R} \right)} \right)}} = {{{- \frac{M}{2}}{\ln\left( \frac{\det\left( \hat{R} \right)}{\left( {\det\left( \hat{\Gamma} \right)} \right)^{2}} \right)}} = {{- \frac{M}{2}}{\ln\left( \frac{\det\left( {\hat{\Gamma} - {\hat{C}{\hat{\Gamma}}^{- *}{\hat{C}}^{*}}} \right)}{\det\left( \hat{\Gamma} \right)} \right)}}}}$where ln( ) indicates the natural logarithm, with L({circumflex over(R)}) indicating the likelihood ratio of the hypotheses of circularityvs. that of non-circularity.

If a structure is present in the (carrier) signal in question, thatgives rise to a constrained covariance estimation problem, thisconstraint should be included in the covariance estimates used forevaluating the likelihood ratio test. In that case, the likelihood ratiotest should be performed based on the constrained covariance matrixestimates and not the sample covariance matrices directly. The simplestexample of such a constrained covariance matrix estimation results whenthe estimation is constrained so that the covariance matrices are knownto be given by a scalar parameter multiplied by an identity matrix. Sucha covariance matrix estimation constraint could for example originatefrom an expectation that the (wireless) communication channel is afrequency-flat channel. When the covariance matrices have this specialstructure, the negative log-likelihood ratio may be found as

$\lambda = {{{- \frac{M}{2}}{\ln\left( \frac{\det\left( {{\hat{\gamma}I} - {\frac{{\hat{c}}^{2}}{\hat{\gamma}}I}} \right)}{\det\left( {\hat{\gamma}I} \right)} \right)}} = {{- \frac{M}{2}}{\ln\left( {1 - \frac{{\hat{c}}^{2}}{{\hat{\gamma}}^{2}}} \right)}}}$with the constrained covariance estimates given by

$\hat{\gamma}\overset{\Delta}{=}{\frac{1}{N}{tr}\left\{ \hat{\Gamma} \right\}}$$\hat{c}\overset{\Delta}{=}{\frac{1}{N}{tr}\left\{ \hat{C} \right\}}$where tr{•} is the trace operator returning the sum of the diagonalelements.

More generally, constrained covariance estimation may be achieved viacommonly known methods of spectral estimation with the resultingconstrained covariance estimates being used as inputs to the likelihoodratio test.

The power measure of such an observation signal, which is a commonmeasure used in conventional carrier ranking, may be represented by anormal RSSI measure, the calculation of which may be expressed by

${RSSI}\overset{\Delta}{=}{{\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{y_{m}}^{2}}} = {{\frac{1}{N}{tr}\left\{ \hat{\Gamma} \right\}} = \hat{\gamma}}}$

Based on a computed RSSI measure and a computed non-circularity measure,a modified ranking procedure may be established according to exemplaryembodiments of the present invention, which is capable of accounting forthe presence of non-circular statistics.

FIG. 2 shows a flowchart illustrating a procedure for calculating anon-circularity measure of an observation signal according to exemplaryembodiments of the present invention. Such procedure may correspond tothe operation 130 according to FIG. 1.

As shown in FIG. 2, a corresponding procedure according to exemplaryembodiments of the present invention comprises an operation of(suitably) preconditioning (210) the observation signal in accordancewith a modulation scheme of the carrier signal, namely the signalstructure of the signal of interest (e.g. a cell search and/orsynchronization signal of a given cellular system), an operation ofestimating (220) non-circular statistics of the preconditionedobservation signal, and an operation of computing (230) anon-circularity measure based on the estimated non-circularitystatistics.

According to exemplary embodiments of the present invention, theaforementioned preconditioning operation is not necessarily required(when the observation signal as such is already suitably(pre-)conditioned. Namely, such preconditioning of the observationsignal may only be performed, if necessary (when the observation signalis to be adapted to the signal structure of the signal of interest).Such preconditioning may for example be accomplished by way of e.g. aderotation for GSM signals or a FFT for LTE signals. Without thepreconditioning being performed, the subsequent operations of estimatingand computing are performed on the observation signal as such (insteadof the preconditioned observation signal).

According to exemplary embodiments of the present invention, theaforementioned computing operation may be based on a likelihood ratiotest as outlined above.

For the subsequent explanation of such a procedure, two examples areassumed. The first example considers a case in which a GSM carriersignal is received at a terminal, and the second example considers acase in which a LTE carrier signal is received at a terminal.

In the following, an exemplary description for the case of a GSM carriersignal is given.

For this example, a version of the observation signal coming from theantenna system, {tilde over (y)}_(m), is derotated according to theGSM-typical GMSK modulation. The preconditioned signal used in thesubsequent processing can therefore be expressed by

$y_{m}\overset{\Delta}{=}{{\mathbb{e}}^{{- j}\frac{\pi}{2}m}{\overset{\sim}{y}}_{m}}$with j being the imaginary unit.

For simplicity, it is here assumed that the observation signal issampled at one sample per symbol (while this is not required). Suchderotation according to the GMSK modulation of the GSM carrier signal iseffective in order to estimate the non-circular statistics of the GMSKcarrier signal in the complex plane. Without such derotation, thenon-circularity thereof could not be seen as the subsequently describedestimation process is to be aligned with the transmission to capture thestationarity of the non-circular process.

Based on this preconditioned observation signal, a measure of thenon-circular statistics may then be estimated by means of a likelihoodratio test based on the covariance matrix {circumflex over (Γ)} and thecomplementary covariance matrix Ĉ, as described in general terms above.

In the following, an exemplary description for the case a LTE carriersignal is given.

In modern wireless systems like LTE, multiple transmit antennas may beemployed allowing the use of transmit diversity schemes for improvedreliability, e.g. Alamouti precoding. Using such transmit diversityprecoding, the (preconditioned) observation signal y_(m) of the precodedblock number m may be described asy _(m) =H(P _(L) x _(m) +P _(CL) x _(m)*)+e _(m)where H represent the MIMO channel transfer function, x_(m) is thevector of data symbols before precoding, P_(L) and P_(CL) are the linearand conjugate-linear precoders, i.e. precoding functions, respectively,and e_(m) is the additive noise.

Since this form of precoding involves sending linear andconjugate-linear versions of the data symbols, the transmitted signalmay exhibit non-circular statistics. In LTE, such a form of transmitdiversity may be used that performs precoding over the space-frequencydomain. The (preconditioned) observation vector y_(m) should thereforebe set up so as to span the space over which the precoding is performed,e.g. two neighboring OFDM sub-carriers.

Based on this observation signal (which may be preconditioned by e.g. aderotation operation or an FFT operation), a measure of the non-circularstatistics may be estimated by means of a likelihood ratio test based onthe covariance matrix {circumflex over (r)} and the complementarycovariance matrix Ĉ, as described in general terms above.

For both exemplary cases described above, the next step is then tocompute a non-circularity measure based on the estimated non-circularitystatistics and to calculate a ranking measure of the carrier basedthereon.

FIG. 3 shows a flowchart illustrating a procedure for calculating aranking measure of a carrier according to exemplary embodiments of thepresent invention. Such procedure may correspond to the operation 140according to FIG. 1.

As shown in FIG. 3, a corresponding procedure according to exemplaryembodiments of the present invention comprises an operation ofdetermining (310), if the desired cellular system has or is expected tohave non-circularity in its carrier signal, i.e. its (carrier) signal ofinterest (e.g. a cell search and/or synchronization signal of a givencellular system), and an operation of calculating (320, 330) the rankingmeasure based on the determination result. In particular, the rankingmeasure is calculated based on an addition of the calculatednon-circularity measure to the calculated power measure (320) when (itis determined that) the desired cellular system has or is expected tohave non-circularity in its carrier signal, i.e. its (carrier) signal ofinterest, and the ranking measure is calculated based on a subtractionof the calculated non-circularity measure from the calculated powermeasure (330) when (it is determined that) the desired cellular systemdoes not have or is not expected to have non-circularity in its carriersignal, i.e. signal of interest.

In the calculation operations 320 and 330 according to FIG. 3, theranking measure may be calculated using a (normalized) addition orsubtraction, respectively.

In the calculation operations 320 and 330 according to FIG. 3, theranking measure may be calculated using a weighting of the calculatedpower measure and the calculated non-circularity measure.

By way of such calculation of the ranking measure, the priority of thecarrier signal under consideration is increased when it originates fromor belongs to the same cellular system (or type, standard, release,specification, etc. thereof) as the cellular system (or type, standard,release, specification, etc. thereof) the terminal desires to connect toor synchronize with, while the priority of the carrier signal underconsideration is decreased when it originates from or belongs to acellular system (or type, standard, release, specification, etc.thereof) other than the cellular system (or type, standard, release,specification, etc. thereof) the terminal desires to connect to orsynchronize with. Thereby, the applicability of the considered carrierfor synchronization with the desired cellular system is reflected in theranking measure according to exemplary embodiments of the presentinvention, and it is thus enabled to prioritize those carriers, whichare more likely to hold valid synchronization signals of the desiredcellular system, over those carriers, which are less likely to holdvalid synchronization signals of the desired cellular system.

In the present example, using the power measure and the non-circularitymeasure being calculated as described above, the ranking measure may becalculated as follows.

In case the desired cellular system has non-circularity in its signal ofinterest (e.g. a cell search and/or synchronization signal of a givencellular system), e.g. the terminal presently considering a carriersignal actually desires to connect to and synchronize with a GSM network(in which the synchronization signal is expected to have non-circularstatistics), the relevant ranking measure RSSI_(non-circular) could forexample be calculated byln(RSSI_(non-circular))=ln(RSSI)+c _(non-circular)·λwherein c_(non-circular) represents a weighting value or constantdetermining how much weight is put onto the non-circularity measure λ.In particular, a value of c_(non-circular)=0 would give the normal RSSImeasure, whereas a large positive value of c_(non-circular) would put ahigh weight on the non-circularity measure.

Similarly, in case the desired cellular system does not havenon-circularity in its signal of interest (e.g. a cell search and/orsynchronization signal of a given cellular system), e.g. the terminalpresently considering a carrier signal actually desires to connect toand synchronize with a (non-GSM) network in which the synchronizationsignal is expected to have circular statistics, the relevant rankingmeasure RSSI_(circular) could for example be calculated byln(RSSI_(circular))=ln(RSSI)−c _(circular)·λwherein c_(circular) represents a weighting value or constantdetermining how much weight is put onto the non-circularity measure λ.In particular, a value of c_(circular)=0 would give the normal RSSImeasure, whereas a large positive value of c_(circular) would put a highweight on the non-circularity measure.

Accordingly, depending on the desired cellular system and the actualcellular system of the carrier under consideration, the carrier isassigned a correspondingly calculated ranking measure.

As mentioned above, an equivalent functionality may be alternativelyaccomplished based on operations working directly on the real andimaginary parts of the signal, i.e. based on an IQ-splitnotation/representation.

In this regard, a non-circularity measure may be computed based on anestimated real-valued covariance matrix of the (preconditioned)observation signal. That is, such methodology basically differs fromthat outlined above in the computation operation illustrated in FIG. 2.

Using an augmented real-valued notation, here termed the IQ-splitnotation/representation, the (preconditioned) observation signal can beexpressed by

${y_{m,{IQ}}\overset{\Delta}{=}\begin{bmatrix}{{Re}\left\{ y_{m} \right\}} \\{{Im}\left\{ y_{m} \right\}}\end{bmatrix}},$wherein Re{.} represents the real part thereof, and Im{.} represents theimaginary part thereof.

The non-circular statistics may be estimated by means of estimating thesample covariance matrix of the IQ-split (preconditioned) observationsignal based on the M observation samples thereof

${\hat{R}}_{IQ} = {\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{y_{m,{IQ}}y_{m,{IQ}}^{T}}}}$

Based on this equivalent IQ-split sample covariance matrix, anon-circularity measure may equivalently be computed in a manner similarto that described above for the complex-valued model, e.g. by alikelihood ratio test or other suitable computations.

One such alternative way of computing a non-circularity measure is basedon the eigenvalues of the estimated covariance matrix. Such a method maybe of particular interest for the case of scalar observations (N=1)where the IQ-split sample covariance matrix is a 2×2 real-valued matrixgiven by

${{\hat{R}}_{IQ} = {{\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{y_{m,{IQ}}y_{m,{IQ}}^{T}}}} = {\begin{bmatrix}{\hat{r}}_{II} & {\hat{r}}_{IQ} \\{\hat{r}}_{IQ} & {\hat{r}}_{QQ}\end{bmatrix} = {V\;\Lambda\; V^{T}}}}},$where V represents the matrix of eigenvectors and Λ=diag([λ₁ λ₂])represents the positive eigenvalues with λ₁≧λ₂. Accordingly, theeigenvalues may be identified thereby.

Based on the eigenvalues, an alternative non-circularity measure of theobservation signal may for example be calculated as λ=ln(λ₁/λ₂).

As second-order matrices (i.e. 2×2 matrices) are relevant and dealt within the presently described example, the ratio of eigenvalues λ₁/λ₂ maybe found by

${\frac{\lambda_{1}}{\lambda_{2}} = \frac{{\hat{r}}_{II} + {\hat{r}}_{QQ} + \sqrt{\left( {{\hat{r}}_{II} + {\hat{r}}_{QQ}} \right)^{2} - {4\;{\det\left( \hat{R} \right)}}}}{{\hat{r}}_{II} + {\hat{r}}_{QQ} - \sqrt{\left( {{\hat{r}}_{II} + {\hat{r}}_{QQ}} \right)^{2} - {4\;{\det\left( \hat{R} \right)}}}}},{{\det\left( {\hat{R}}_{IQ} \right)} = {{{\hat{r}}_{II}{\hat{r}}_{QQ}} - {\hat{r}}_{IQ}^{2}}},$wherein det(.) represents the determinant operator.

FIG. 4 shows a flowchart illustrating a cell search/synchronizationprocedure according to exemplary embodiments of the present invention.

As shown in FIG. 4, a corresponding procedure according to exemplaryembodiments of the present invention comprises an operation of receiving(410) a plurality of carrier signals on a plurality of carriers, anoperation of carrying out (420) the above-described acquiring andcalculating operations for each of the plurality of carrier signals(which is represented by the loop in FIG. 4), an operation of ranking(430) the plurality of carriers in accordance with their applicabilityfor synchronization with the desired cellular system based on thecalculated ranking measures of said carriers, and an operation ofexecuting (440) a cell search and/or synchronization operation forsynchronization with the desired cellular system based on the carrierranking starting with the carrier having the highest ranking.

The operation 420 according to FIG. 4 may correspond to the procedureaccording to FIG. 1, including operations 110 to 140, and potentiallythe respective procedures according to FIGS. 2 and/or 3, respectively.

The operation 440 according to FIG. 4 may comprise conventionally knownoperations for the execution of cell search and synchronization, such asone or more of PSW detection, SCH detection, and BSIC verification forGSM as well as P-SCH detection, S-SCH detection and PBCH detection forLTE. Namely, based on the prioritized list of ranked carriers inaccordance with the proposed ranking measure according to exemplaryembodiments of the present invention, the next step of initialsynchronization may then be to perform such steps starting with thecarrier with the highest ranking (measure).

Accordingly, all of the thus received carrier signals are successivelyconsidered for synchronization, respectively. Then, their thuscalculated ranking measures are used for establishing a ranking of theplurality of carriers, and the cell search and synchronization isexecuted based on such carrier ranking.

By virtue of the advanced ranking measure and the thus resultingimproved ranking according to exemplary embodiments of the presentinvention, the likelihood of a failed attempt for synchronization on awrong carrier (i.e. with an inappropriate synchronization signal) isreduced. This is basically achieved by incorporation of knowledge ofnon-/circularity properties of carrier signal.

In the above description, it is exemplarily assumed that the observationsignal is sampled at one sample per data symbol. Yet, exemplaryembodiments of the present invention are not limited thereby, and theobservation signal may equally be sampled at an arbitrary (integer)number of samples per data symbol. Thereby, the detection ofnon-circular statistics may potentially be further improved.

In the above description, it is exemplarily assumed that the covariancematrix of the observation signal assumes a frequency-flat channel. Yet,exemplary embodiments of the present invention are not limited thereby,as covariance matrix estimation for frequency-selective channels mayequally be involved. Thereby, the detection of non-circular statisticsmay be further improved.

The above-described procedures and functions may be implemented byrespective functional elements, processors, or the like, as describedbelow.

While in the foregoing exemplary embodiments of the present inventionare described mainly with reference to methods, procedures andfunctions, corresponding exemplary embodiments of the present inventionalso cover respective apparatuses, network nodes and systems, includingboth software and/or hardware thereof.

Respective exemplary embodiments of the present invention are describedbelow referring to FIG. 5, while for the sake of brevity reference ismade to the detailed description of respective corresponding methods andoperations according to FIGS. 1 to 4.

In FIG. 5 below, which is noted to represent a simplified block diagram,the solid line blocks are basically configured to perform respectiveoperations as described above. The entirety of solid line blocks arebasically configured to perform the methods and operations as describedabove, respectively. With respect to FIG. 5, it is to be noted that theindividual blocks are meant to illustrate respective functional blocksimplementing a respective function, process or procedure, respectively.Such functional blocks are implementation-independent, i.e. may beimplemented by means of any kind of hardware or software, respectively.The arrows and lines interconnecting individual blocks are meant toillustrate an operational coupling there-between, which may be aphysical and/or logical coupling, which on the one hand isimplementation-independent (e.g. wired or wireless) and on the otherhand may also comprise an arbitrary number of intermediary functionalentities not shown. The direction of arrow is meant to illustrate thedirection in which certain operations are performed and/or the directionin which certain data is transferred.

Further, in FIG. 5, only those functional blocks are illustrated, whichrelate to any one of the above-described methods, procedures andfunctions. A skilled person will acknowledge the presence of any otherconventional functional blocks required for an operation of respectivestructural arrangements, such as e.g. a power supply, a centralprocessing unit, respective memories or the like. Among others, memoriesare provided for storing programs or program instructions forcontrolling the individual functional entities to operate as describedherein.

FIG. 5 shows a block diagram illustrating an apparatus and systemaccording to exemplary embodiments of the present invention.

In view of the above, the thus described apparatuses 10 and 20 aresuitable for use in practicing the exemplary embodiments of the presentinvention, as described herein. The thus described apparatus 10 mayrepresent a (part of a) terminal such as a mobile station MS or userequipment UE or a modem (which may be installed as part of a MS or UE,but may be also a separate module a chip or a chipset, which can beattached to various devices), as described above, and may be configuredto perform a procedure and/or exhibit a functionality as described inconjunction with any one of FIGS. 1 to 4. The thus described apparatus20 may represent a (part of a) network entity being operable on at leastone cellular system, i.e. base station BS, a base transceiver stationBTS or an access node, such as for example a NodeB, an eNB, or the like,and may be configured to transmit (e.g. broadcast) one or more carriersignals on one or more carriers, and to connect a terminal to the atleast one cellular system in the context of a terminal-initiated cellsearch and synchronization procedure.

As shown in FIG. 5, according to exemplary embodiments of the presentinvention, a terminal 10 comprises a processor 11, a memory 12, and aninterface 13, which are connected by a bus 14 or the like. The terminal10 may be in communication with a base (transceiver) station 20 througha link or connection 30.

The memory 12 may store respective programs assumed to include programinstructions or computer program code that, when executed by theprocessors 11, enables the respective electronic device or apparatus tooperate in accordance with the exemplary embodiments of the presentinvention.

The processor 11 and/or the interface 13 may be facilitated forcommunication over a (hardwire or wireless) link, respectively. Theinterface 13 may include a suitable transceiver coupled to one or moreantennas or communication means for (hardwire or wireless)communications with the linked or connected device(s), respectively. Theinterface 13 is generally configured to communicate with at least oneother apparatus, i.e. the interface thereof. For example, the interface13 of the terminal 10 may communicate with one or more other networkentities such as base (transceiver) station 20.

In general terms, the respective devices/apparatuses (and/or partsthereof) may represent means for performing respective operations and/orexhibiting respective functionalities, and/or the respective devices(and/or parts thereof) may have functions for performing respectiveoperations and/or exhibiting respective functionalities.

When in the subsequent description it is stated that the processor (orsome other means) is configured to perform some function, this is to beconstrued to be equivalent to a description stating that at least oneprocessor, potentially in cooperation with computer program code storedin the memory of the respective apparatus, is configured to cause theapparatus to perform at least the thus mentioned function. Also, suchfunction is to be construed to be equivalently implementable byspecifically configured means for performing the respective function(i.e. the expression “processor configured to [cause the apparatus to]perform xxx-ing” is construed to be equivalent to an expression such as“means for xxx-ing”).

According to exemplary embodiments of the present invention, anapparatus representing the terminal 10 comprises at least one processor11, at least one memory 12 including computer program code, and at leastone interface 13 configured for communication with at least anotherapparatus. The processor (i.e. the at least one processor 11, with theat least one memory 12 and the computer program code) is configured tocause the apparatus 10 to perform: acquiring an observation signal for acarrier signal on a carrier which is under consideration forsynchronization with a desired cellular system, calculating a powermeasure of the observation signal, which indicates a received power ofthe carrier signal, calculating a non-circularity measure of theobservation signal, which indicates a non-circularity of the carriersignal, and calculating a ranking measure, which indicates anapplicability of said carrier for synchronization with the desiredcellular system, based on the calculated power measure and thecalculated non-circularity measure.

As mentioned above, the desired cellular system and/or the actualcellular system may comprise one or more of GSM, 3G, 3.5G, CDMA, WCDMA,TD-SCDMA, HSPA, LTE, and LTE-A cellular systems. Stated in other words,the terminal (and/or the communicating base station/s) may be operablein at least one of GSM, 3G, 3.5G, CDMA, WCDMA, TD-SCDMA, HSPA, LTE, andLTE-A cellular systems.

According to exemplary embodiments of the present invention, theprocessor 11 may be configured to perform: preconditioning theobservation signal in accordance with a modulation scheme of the carriersignal, estimating non-circular statistics of the preconditionedobservation signal, if necessary, and computing a non-circularitymeasure based on the estimated non-circular statistics. Therein, theestimated non-circular statistics may comprise an estimation ofcovariance matrices of said (possibly preconditioned) observationsignal, and the computed non-circularity measure may be based on alikelihood ratio test on the basis of the estimated covariance matrices.Further, the estimated non-circular statistics may also compriseimposing suitable constraints on the covariance matrix estimation basedon properties of expected channel conditions, e.g. a frequency-flatchannel. That is, when preconditioning is performed, the estimation andcomputation operations are performed on the preconditioned observationsignal, while, when preconditioning is not performed, the estimation andcomputation operations are performed on the observation signal as such.

According to exemplary embodiments of the present invention, theprocessor 11 may be configured to compute the non-circularity measurebased on eigenvalues of the estimated non-circular statistics or a ratiothereof. In this regard, the processor 11 may be further configured toidentify the eigenvalues of the estimated non-circular statistics.

According to exemplary embodiments of the present invention, theprocessor 11 may be configured to perform: determining, ifnon-circularity of said carrier signal is expected to be present in thedesired cellular system, and calculating the ranking measure based on anaddition of the calculated non-circularity measure to the calculatedpower measure when it is determined that the desired cellular system isexpected to have non-circular statistics in its carrier signal, and/orcalculating the ranking measure based on a subtraction of the calculatednon-circularity measure from the calculated power measure when it isdetermined that the desired cellular system is not expected to havenon-circular statistics in its carrier signal. Therein, the rankingmeasure may be calculated using a weighting of the calculated powermeasure and the calculated non-circularity measure.

According to exemplary embodiments of the present invention, theprocessor 11 may be configured to perform: carrying out the acquiringand calculating operations for a plurality of carrier signals on aplurality of carriers, ranking the plurality of carriers in accordancewith their applicability for synchronization with the desired cellularsystem based on the calculated ranking measures of the carriers, andexecuting a cell search and/or synchronization operation forsynchronization with the desired cellular system based on the carrierranking starting with the carrier having the highest ranking.

According to exemplary embodiments of the present invention, the memory12 may be configured to store any data and information received via theinterface 13, and/or any data and information necessary for (enablingthe processor to carry out) the aforementioned operations and functions.For example, the memory 12 may contain (e.g. a database with) prestoredproperties of cellular systems (in particular, those in/for which theterminal is operable and/or those having carriers coexisting on the samefrequency range/band as that of the cellular system in/for which theterminal is operable). Such properties of cellular systems mayspecifically be configured to enable a distinction between (mapping of)carrier signals and/or synchronization channels of different cellularsystem. In this regard, such properties of cellular systems may compriseproperties in terms of a modulation scheme used for carrier signalsand/or synchronization channels, a non-/circularity (e.g. non-/circularstatistics in the complex plane and/or from precoding such as due totransmit diversity schemes) of carrier signals and/or synchronizationchannels, and the like.

According to exemplary embodiments of the present invention, theprocessor 11, the memory 12 and the interface 13 can be implemented asindividual modules, chips, chipsets or the like, or one or more of themcan be implemented as a common module, chip, chipset or the like,respectively.

According to exemplary embodiments of the present invention, a systemmay comprise any conceivable combination of the thus depicteddevices/apparatuses and other network elements, which are configured tocooperate as described above.

In general, it is to be noted that respective functional blocks orelements according to above-described aspects can be implemented by anyknown means, either in hardware and/or software, respectively, if it isonly adapted to perform the described functions of the respective parts.The mentioned method steps can be realized in individual functionalblocks or by individual devices, or one or more of the method steps canbe realized in a single functional block or by a single device.

Generally, any method step is suitable to be implemented as software orby hardware without changing the idea of the present invention. Suchsoftware may be software code independent and can be specified using anyknown or future developed programming language, such as e.g. Java, C++,C, and Assembler, as long as the functionality defined by the methodsteps is preserved. Such hardware may be hardware type independent andcan be implemented using any known or future developed hardwaretechnology or any hybrids of these, such as MOS (Metal OxideSemiconductor), CMOS (Complementary MOS), BiMOS (Bipolar MOS), BiCMOS(Bipolar CMOS), ECL (Emitter Coupled Logic), TTL (Transistor-TransistorLogic), etc., using for example ASIC (Application Specific IC(Integrated Circuit)) components, FPGA (Field-programmable Gate Arrays)components, CPLD (Complex Programmable Logic Device) components or DSP(Digital Signal Processor) components. A device/apparatus may berepresented by a semiconductor chip, a chipset, or a (hardware) modulecomprising such chip or chipset; this, however, does not exclude thepossibility that a functionality of a device/apparatus or module,instead of being hardware implemented, be implemented as software in a(software) module such as a computer program or a computer programproduct comprising executable software code portions for execution/beingrun on a processor. A device may be regarded as a device/apparatus or asan assembly of more than one device/apparatus, whether functionally incooperation with each other or functionally independently of each otherbut in a same device housing, for example.

Devices and means can be implemented as individual devices, but thisdoes not exclude that they are implemented in a distributed fashionthroughout the system, as long as the functionality of the device ispreserved. Such and similar principles are to be considered as known toa skilled person.

Software in the sense of the present description comprises software codeas such comprising code means or portions or a computer program or acomputer program product for performing the respective functions, aswell as software (or a computer program or a computer program product)embodied on a tangible medium such as a computer-readable (storage)medium having stored thereon a respective data structure or codemeans/portions or embodied in a signal or in a chip, potentially duringprocessing thereof.

The present invention also covers any conceivable combination of methodsteps and operations described above, and any conceivable combination ofnodes, apparatuses, modules or elements described above, as long as theabove-described concepts of methodology and structural arrangement areapplicable.

In view of the above, the present invention and/or exemplary embodimentsthereof provide measures for cell search and synchronization. Suchmeasures may exemplarily comprise acquiring an observation signal for acarrier signal on a carrier which is under consideration forsynchronization with a desired cellular system, calculating a powermeasure of the observation signal, which indicates a received power ofsaid carrier signal, calculating a non-circularity measure of theobservation signal, which indicates a non-circularity of said carriersignal, and calculating a ranking measure, which indicates anapplicability of said carrier for synchronization with the desiredcellular system, based on the calculated power measure and thecalculated non-circularity measure.

Even though the present invention and/or exemplary embodiments aredescribed above with reference to the examples according to theaccompanying drawings, it is to be understood that they are notrestricted thereto. Rather, it is apparent to those skilled in the artthat the present invention can be modified in many ways withoutdeparting from the scope of the inventive idea as disclosed herein.

List of acronyms and abbreviations 3GPP 3rd Generation PartnershipProject AWGN Additive White Gaussian Noise BPSK Binary Phase ShiftKeying BS Base Station BSIC Base Station Identity Code BTS BaseTransceiver Station CDMA Code Division Multiple Access eNB evolved NodeBFFT Fast Fourier Transform GLRT Generalized Likelihood Ratio Test GMSKGaussian Minimum Shift Keying GSM Global System for Mobilecommunications HSPA High Speed Packet Access IEEE Institute ofElectrical and Electronics Engineers LTE Long Term Evolution LTE-A LongTerm Evolution Advanced MIMO Multiple Input Multiple Output MS MobileStation OFDM Orthogonal Frequency Division Multiplexing PBCH PhysicalBroadcast CHannel P-SCH Primary SCH PSW Pure Sine Wave RSSI ReceivedSignal Strength Indicator SCH Synchronization Channel S-SCH SecondarySCH TD-SCDMA Time Division Synchronous Code Division Multiple Access UEUser Equipment WCDMA Wideband Code Division Multiple Access

What is claimed is:
 1. A method comprising acquiring an observationsignal for a carrier signal on a carrier which is under considerationfor synchronization with a desired cellular system, calculating a powermeasure of the observation signal, which indicates a received power ofsaid carrier signal, calculating a non-circularity measure of theobservation signal, which indicates a non-circularity of said carriersignal, and calculating a ranking measure, which indicates anapplicability of said carrier for synchronization with the desiredcellular system, based on the calculated power measure and thecalculated non-circularity measure.
 2. The method according to claim 1,wherein calculating the non-circularity measure of the observationsignal comprises estimating non-circular statistics of said observationsignal, and computing a non-circularity measure based on the estimatednon-circular statistics.
 3. The method according to claim 2, whereincalculating the non-circularity measure of the observation signalfurther comprises preconditioning said observation signal in accordancewith a modulation scheme of said carrier signal, wherein thenon-circular statistics of the preconditioned observation signal areestimated.
 4. The method according to claim 2, wherein the estimatednon-circular statistics comprises an estimation of covariance matricesof said observation signal, and the computed non-circularity measure isbased on a likelihood ratio test on the basis of the estimatedcovariance matrices.
 5. The method according to claim 2, wherein thenon-circularity measure is computed based on eigenvalues of theestimated non-circular statistics or a ratio thereof.
 6. The methodaccording to claim 1, wherein calculating the ranking measure comprisesdetermining, when non-circularity of said carrier signal is expected tobe present in the desired cellular system, and calculating the rankingmeasure based on an addition of the calculated non-circularity measureto the calculated power measure when it is determined that the desiredcellular system is expected to have non-circular statistics in itscarrier signal, and calculating the ranking measure based on asubtraction of the calculated non-circularity measure from thecalculated power measure when it is determined that the desired cellularsystem is not expected to have non-circular statistics in its carriersignal, wherein the ranking measure is calculated using a weighting ofthe calculated power measure and the calculated non-circularity measure.7. The method according to claim 1, further comprising carrying out theacquiring and calculating operations for a plurality of carrier signalson a plurality of carriers, ranking the plurality of carriers inaccordance with their applicability for synchronization with the desiredcellular system based on the calculated ranking measures of saidcarriers, and executing a cell search and/or synchronization operationfor synchronization with the desired cellular system based on thecarrier ranking starting with the carrier having the highest ranking. 8.The method according to claim 1, wherein non-circularity of said carriersignal exists in the complex plane or any other space over which signalprecoding is done, and/or said carrier signal comprises asynchronization signal of the actual cellular system of said carrier,and/or said carrier signal has a frequency within a predeterminedfrequency range of the desired cellular system, wherein saidpredetermined frequency range is configurable to accommodate multiplecellular systems or carrier signals thereof, and/or the calculated powermeasure comprises a received signal strength indicator value of saidobservation signal.
 9. The method according to claim 1, wherein themethod is operable at or by a terminal, user equipment or mobile stationbeing operable at least in the desired cellular system, and/or thedesired cellular system and the actual cellular system comprise one ormore of GSM, 3G, 3.5G, CDMA, WCDMA, TD-SCDMA, HSPA, LTE, and LTE-Acellular systems.
 10. An apparatus comprising at least one processor, atleast one memory including computer program code, and at least oneinterface configured for communication with at least another apparatus,the at least one processor, with the at least one memory and thecomputer program code, being configured to cause the apparatus toperform: acquiring an observation signal for a carrier signal on acarrier which is under consideration for synchronization with a desiredcellular system, calculating a power measure of the observation signal,which indicates a received power of said carrier signal, calculating anon-circularity measure of the observation signal, which indicates anon-circularity of said carrier signal, and calculating a rankingmeasure, which indicates an applicability of said carrier forsynchronization with the desired cellular system, based on thecalculated power measure and the calculated non-circularity measure. 11.The apparatus according to claim 10, the at least one processor, withthe at least one memory and the computer program code, being furtherconfigured to cause the apparatus to perform: estimating non-circularstatistics of said observation signal, and computing a non-circularitymeasure based on the estimated non-circular statistics.
 12. Theapparatus according to claim 11, the at least one processor, with the atleast one memory and the computer program code, being further configuredto cause the apparatus to perform preconditioning said observationsignal in accordance with a modulation scheme of said carrier signal,wherein the non-circular statistics of the preconditioned observationsignal are estimated.
 13. The apparatus according to claim 11, whereinthe estimated non-circular statistics comprises an estimation ofcovariance matrices of said observation signal, and the computednon-circularity measure is based on a likelihood ratio test on the basisof the estimated covariance matrices.
 14. The apparatus according toclaim 11, wherein the non-circularity measure is computed based oneigenvalues of the estimated non-circular statistics or a ratio thereof.15. The apparatus according to claim 10, the at least one processor,with the at least one memory and the computer program code, beingfurther configured to cause the apparatus to perform: determining, whennon-circularity of said carrier signal is expected to be present in thedesired cellular system, and calculating the ranking measure based on anaddition of the calculated non-circularity measure to the calculatedpower measure when it is determined that the desired cellular system isexpected to have non-circular statistics in its carrier signal, andcalculating the ranking measure based on a subtraction of the calculatednon-circularity measure from the calculated power measure when it isdetermined that the desired cellular system is not expected to havenon-circular statistics in its carrier signal, wherein the at least oneprocessor, with the at least one memory and the computer program code,being further configured to cause the apparatus to calculate the rankingmeasure using a weighting of the calculated power measure and thecalculated non-circularity measure.
 16. The apparatus according to claim10, the at least one processor, with the at least one memory and thecomputer program code, being further configured to cause the apparatusto perform: carrying out the acquiring and calculating operations for aplurality of carrier signals on a plurality of carriers, ranking theplurality of carriers in accordance with their applicability forsynchronization with the desired cellular system based on the calculatedranking measures of said carriers, and executing a cell search and/orsynchronization operation for synchronization with the desired cellularsystem based on the carrier ranking starting with the carrier having thehighest ranking.
 17. The apparatus according to claim 10, whereinnon-circularity of said carrier signal exists in the complex plane orany other space over which signal precoding is done, and/or said carriersignal comprises a synchronization signal of the actual cellular systemof said carrier, and/or said carrier signal has a frequency within apredetermined frequency range of the desired cellular system, whereinsaid predetermined frequency range is configurable to accommodatemultiple cellular systems or carrier signals thereof, and/or thecalculated power measure comprises a received signal strength indicatorvalue of said observation signal.
 18. The apparatus according to claim10, wherein the apparatus is operable as or at a terminal, userequipment, mobile station or modem being operable at least in thedesired cellular system, and/or the desired cellular system and theactual cellular system comprise one or more of GSM, 3G, 3.5G, CDMA,WCDMA, TD-SCDMA, HSPA, LTE, and LTE-A cellular systems.
 19. A computerprogram product comprising a non-transitory computer readable storagemedium configured to store computer-executable computer program codewhich, when the program code is run on a computer, is configured tocause the computer to carry out the method according to claim 1.